There are generally two types of remote sensing technologies: passive sensing and active sensing. In passive sensing, images or other representations of a target are created by detecting radiation that is generated by an external source, such as the sun. In contrast, active sensing technologies not only detect radiation reflected or scattered by a target but also generate the radiation to illuminate the target for subsequent detection.
Light Detection and Ranging (LIDAR, also known as LADAR) is an active sensing technique that involves emitting light (e.g., pulses from a laser) and detecting the reflected or scattered light. LIDAR typically measures the time-of-flight (i.e., the time it takes for the pulse to travel from the transmitter to the target, be reflected, and travel back to the sensor), which can be used to derive ranges (or distances) to the target which reflects or scatters the light. In this manner, LIDAR is analogous to radar (radio detecting and ranging), except that LIDAR is based on optical waves instead of radio waves.
LIDAR can be airborne or ground-based. Airborne LIDAR typically collects data from airplanes looking down and covering large areas of the ground. LIDAR can also be conducted from ground-based stationary and mobile platforms. Ground-based LIDAR techniques can be beneficial in producing high accuracies and point densities, thus permitting the development of precise, realistic, three-dimensional representations of scenes such as railroads, roadways, bridges, buildings, breakwaters, or shoreline structures.